1. Field of the Invention
The present invention relates generally to cardiac pacing therapy. More particularly, the present invention relates to a system and method for providing atrial pacing in order to prevent the immediate reentry into fibrillation following the application of an atrial arrhythmia termination therapy.
2. Related Art
Atrial fibrillation is a common cardiac arrhythmia in which small areas of atrial tissue repeatedly depolarize in a disorderly fashion when compared to neighboring areas of atrial tissue. Although it is not normally a life-threatening arrhythmia, it is well-known that atrial arrhythmias lead to dizziness resulting from reduced cardiac output, palpitations of the heart, and even stroke. Atrial fibrillation may be corrected by an external defibrillator configured to apply electrical energy to the heart through the skin of a patient, or through use of an implantable device applying electrical energy to the heart based upon detected R waves produced by the heart. Cardioversion, for example, is a conventional type of post-atrial arrhythmia termination therapy used to treat atrial fibrillation.
A problem with the conventional treatment of atrial fibrillation, also known as atrial therapy, is that the treatments are often followed by an early return to atrial fibrillation (ERAF) after a few cardiac sinus intervals. ERAF reduces the success rate of the pacing therapy and forces repeated and higher-amplitude shocks resulting in additional patient discomfort. The cause of ERAF is not completely understood. It is hypothesized, however, that it might be related to a time-delay in the remodeling of the effective refractive period (ERP) and conduction velocity of the atria. Other hypotheses suggest that it could also be related to a level of dispersion of ERP that exists during atrial fibrillation that might still be present within the first few minutes following cardioversion.
A number of systems have been presented in an attempt to treat various effects of atrial fibrillation. For example, U.S. Pat. No. 5,527,345 issued to Infinger discloses an atrial defibrillator for applying cardioverting electrical energy to the atria of a human heart in need of cardioversion and thereafter pacing the heart in a demand mode. In particular, Infinger relates to the pacing of the ventricles at a base rate following cardioversion to treat sinus node dysfunction, which can lead to dizziness or possible ventricular arrhythmia.
Similarly, U.S. Pat. No. 5,265,600 issued to Adams et al, discloses an implantable atrial defibrillator for applying cardioverting electrical energy to the atria of a human heart in need of atrial therapy and thereafter gradually returning the cardiac rate of the heart to a normal cardiac rate. Although Adams et al. and Infinger address treatment of various aspects of atrial fibrillation, neither discloses a method or technique for treating reinitiation of atrial fibrillation.
Next, U.S. Pat. No. 5,645,569 issued to Ayers, (the '569 patent) discloses an atrial cardioverter/defibrillator and method, wherein after each application of atrial cardioversion therapy, the atria are paced to prevent spontaneous reversion from a normal sinus rhythm back to atrial defibrillation.
Finally, U.S. Pat. No. 5,676,687, issued to Ayers (the '687 patent) discloses an atrial cardioverter defibrillator and method, wherein the atria are paced from a relatively high rate to a gradually derived normal or bradycardia rate to prevent spontaneous reversion from a normal sinus rhythm back to atrial fibrillation. The '687 patent and the '569 patent both address pacing following cardioversion to prevent arrhythmia reinitiation. The '569 patent, however, primarily addresses current clinical pacing modes such as atrial pacing (AAI) and dual-chamber pacing (DDD).
AAI atrial pacing is inhibited by sensing in the atria and DDD pacing is inhibited by sensing in the atria or ventricle. The '687 patent discloses a technique by which the atria are paced in a demand mode at a rate greater than the brady rate, followed by a gradual decrease to a bradycardia rate. Both the '569 patent and the '687 patent allow for an initial unsynchronized pacing pulse that may be conducive to initiating an arrhythmia.
The '569 and '687 patents discuss pacing either in the right atria, the left atria, or pacing between the atria. Both patents also call for pacing termination at the end of a fixed time interval or a certain number of sequential sense events. Conventional techniques have not been shown to be particularly effective at preventing reinitiation of atrial fibrillation following cardioversion therapy.
What is needed, therefore, is a system and method for preventing ERAF following the application of post-atrial arrhythmia therapy.